1. Field of the Invention
The present invention relates to a nondestructive test method for quantitatively determining fatigue of a ferromagnetic construction material, or of a structure comprised of such a material.
2. Description of the Related Art
Conventional nondestructive test methods for determining fatigue of materials are generally based on investigation of generation and growth of cracks in the material, and thus, it is highly important to find out as minute cracks as possible. With such a conventional nondestructive test method, it is practically impossible to evaluate fatigue of the material before cracks are generated.
There is known another type of nondestructive fatigue test method that is applicable to ferromagnetic construction materials or structures comprised of such a construction material. In this test method, the coercive force and magnetic susceptibility of the test material are measured in the range approaching to saturation. In this instance, for precisely determining the coercive force of the test material, it is necessary to provide a magnetizing yoke and a winding coil around it such that the test material can be magnetized to a saturation level and then demagnetized until the internal magnetic flux becomes zero. To this end, a magnetic force has to be applied that is far larger than the coercive force of that material, by using a large magnetizing yoke and allowing a large magnetizing current to flow through the magnetizing coil. A test machine incorporating such a large magnetic yoke and a large capacity magnetizing power source for energizing the magnetic yoke is not only expensive, but also makes the entire system heavy and large in size to require a noticeable installation space.
It is therefore a primary object of the present invention to provide an improved test method for nondestructively determining the fatigue of a ferromagnetic construction material, which advantageously eliminates the above-mentioned problems of the prior art.
One aspect of the present invention resides in a method for nondestructively determining fatigue of a test ferromagnetic construction material having a known, initial tensile stress "sgr"0, by quantifying a change in effective stress due to aging of the material. The test method according to the present invention comprises the following four steps.
The first step is to measure a magnetic susceptibility "khgr"c of the test material in its aged state, under a magnetic field having a predetermined intensity H, according to a relation as expressed by a first equation:
c="khgr"c H3xe2x80x83xe2x80x83(1).
The second step is to determine a susceptibility coefficient c of the test material by putting the magnetic field intensity H and the measured magnetic susceptibility "khgr"c of the test material into the first equation.
The third step is to determine a current tensile stress "sgr" of the test material, by putting the value of the susceptibility coefficient c into a second equation:
"sgr"={log (c)xe2x88x92a})bxe2x80x83xe2x80x83(2)
where a and b are known constants determined by a structure of the test material.
Finally, the fourth step is to determine a change in effective tensile stress of the test material, by comparing the current tensile stress "sgr" of the test material with the initial tensile stress "sgr"0 of the test material.
Another aspect of the present invention resides in an apparatus nondestructively determining fatigue of a test ferromagnetic construction material having a known, initial tensile stress ("sgr"0), by quantifying a change in the effective stress due to aging of the test material. The apparatus according to the present invention comprises:
i) measuring means for measuring the magnetic susceptibility ("khgr"c) of the test material in its aged state, under a magnetic field of a specified intensity (H), according to a relation as expressed by a first equation:
c="khgr"c H3xe2x80x83xe2x80x83(1)
ii) stress calculation means for calculating and thereby determining a current tensile stress ("sgr") of the test material, by determining a susceptibility coefficient (c) of the test material after putting the measured magnetic susceptibility ("khgr"c) of the test material and the magnetic field intensity (H) into the first equation, and then putting the susceptibility coefficient (c) into the second equation:
"sgr"={log (c)xe2x88x92a}/bxe2x80x83xe2x80x83(2)
where a and b are known constants determined by an internal structure of the test material; and
iii) evaluation means for determining a change in effective stress of the test material due to aging thereof, by comparing the current tensile stress ("sgr") of the test material with its initial tensile stress ("sgr"0).
The nondestructive test apparatus according to the present invention, as a whole, may be comprised of a personal computer installed with programs based on the algorithm which enables execution of the above steps.
The principle of the present invention will be described below with reference to experimental test data. To elucidate the interrelationship between the mechanical and magnetic properties of steel materials, test materials were prepared which consist of single crystal pure iron, polycrystalline pure iron, and low-alloy steel A533B, respectively. These test materials were formed into samples having shapes as shown in FIGS. 1(a), 1(b) and 1(c), respectively, which are to be subjected to tensile and hysteresis loop tests. The material formed into a sample as shown in FIG. 1(a) was used for the tensile test, while the material for med into a sample as shown in FIG. 1(b) or 1(c) was used for the hysteresis loop test. As for the hysteresis loop test, the polycrystalline pure iron and low-alloy steel A533B took the shape of FIG. 1(b) while the single crystal pure iron took the shape of FIG. 1(c). Table 1 below shows the composition of the low-alloy steel A533B submitted to the test.
FIGS. 2 to 4 illustrate the stress-strain characteristics of the test samples, obtained from the tensile test. FIG. 2 represents the results from an Fe since crystal sample, and shows that the strain rate (i.e., extension rate) is 1.5%/min. FIG. 3 represents the results from an Fe polycrystalline sample, and shows that the strain rate is 1.2%/min. FIG. 4 represents the results from an alloy steel A533B sample, and shows that the strain rate is 1.2%/min.
FIGS. 5 and 6 illustrate the magnetization curves obtained from the hysteresis loop test under application of stresses. FIG. 5 shows the hysteresis loop characteristics of an Fe single crystal sample with plastic deformation under a stress (0 MPa, 55 MPa, or 115 MPa), while FIG. 6 shows the hysteresis loop characteristics of an Fe polycrystalline sample with plastic deformation under a stress (0 MPa, 550 MPa, or 663 MPa). The stresses applied were chosen to be equal to 0 MPa and the stress that develops just before breakage, both of which had been obtained from a preparatory tensile test, and to intermediate values between these two values.
From the gradient of the magnetization curve of a test material as depicted in FIGS. 5 and 6, it is possible to determine the magnetization susceptibility "khgr"c of the test material at a magnetic field intensity exceeding its coercive force. FIG. 7 illustrates the relationship of the magnetic susceptibility of the low-alloy steel A533B with the magnetic field intensity H above the coercive force of the material obtained from the magnetization curve under a stress of 663 MPa as depicted in FIG. 6. Similarly, FIG. 8 illustrates the relationship of the logarithmic magnetic susceptibility (log "khgr"c) of the low-alloy steel A533B with the logarithmic magnetic field intensity H (log H) obtained from the magnetic susceptibility curve "khgr"c which changes as a function of the magnetic field intensity H under the stress of 663 MPa as depicted in FIG. 7. Finally, FIG. 9 illustrates the relationship of the logarithmic magnetic susceptibility (log "khgr"c) of an Fe single crystal material with the logarithmic magnetic field intensity H (log H) under the stress of 115 MPa. The straight line indicates the relation between the magnetic susceptibility "khgr"c and the magnetic field intensity H in the equation (1).
From the curves in FIGS. 8 and 9, the following equation (3) can be obtained:
log "khgr"c=xe2x88x923 log H+Axe2x80x83xe2x80x83(3),
where A is a constant. The equation (3) can be transformed into the following equation (4):
"khgr"c=c/H3xe2x80x83xe2x80x83(4).
It is noted that the equation (4) is equivalent to the equation (1) explained above.
In the equation (1) or (4), the factor c is a parameter representing a state of material that is determined by dislocations or other lattice defects, and grain boundaries existent in the same material, and defined as the xe2x80x9csusceptibility coefficientxe2x80x9d. Existence of such susceptibility coefficient c has been known through tests where a magnetic field having a strong intensity is applied to a single crystal material. However, experiments performed by the inventors revealed for the first time that the susceptibility coefficient c exists also in single crystal pure iron, polycrystalline pure iron and low-alloy steel which are exposed to a magnetic field having a relatively low intensity.
With the equation (1): c="khgr"c H3 obtained from the hysteresis of the test materials subjected to the hysteresis loop test, it is possible to plot the susceptibility coefficient c of the materials as a function of the intensity of a magnetic field above the coercive force of the materials, and to further plot the logarithmic values of c as a function of the applied stresses, which gives the results as shown in FIG. 10. The solid triangles (▴), solid circles (xe2x97xaf) and solid diamonds (♦) represent the results obtained from an Fe single crystal material, an Fe polycrystalline material, and a low-alloy steel material, respectively. It has been revealed as a result of investigations by the inventors, that the relation of susceptibility coefficient c with stress "sgr" can be expressed by a single equation (5):
"sgr"={log (c)xe2x88x92a}/bxe2x80x83xe2x80x83(5),
where a and b are constants determined by the internal, crystal structure of the test material. The single crystal pure iron, polycrystalline pure iron, and low-alloy A533B steel submitted to the test each has a body-centered cubic (BCC) lattice structure, and contains iron atoms as main ingredient. Thus, the characteristic under study observed among those materials can be represented by a single line as shown in FIG. 10 which is expressed by the equation (5).
Therefore, even for a sample whose stress is unknown, it is possible to nondestructively determine the current stress "sgr" by resorting to the hysteresis loop test, after determining the susceptibility coefficient c and putting the result into the equation (5). The stress "sgr" serves as a parameter representing the mechanical strength of the material.
It is possible to determine the susceptibility coefficient c of a test material by using a magnetic yoke or winding coil and by nondestructively measuring its hysteresis characteristics. Since what is necessary in this test is only to determine the magnetic susceptibility of the material, it is possible to reduce the intensity of magnetic field to a far lower level than is required in the conventional test based on the measurement of a coercive force, and hence to reduce the magnetizing current to a far lower level than is required in such a conventional test.
Therefore, with the method according to the present invention, it is possible to precisely determine the current stress of a test material by exposing it to a magnetic field with a far less intensity than is required for the conventional method, and thereby nondestructively determine fatigue of the test material. It is to be noted that when a construction material is aged, i.e., exposed to a stress over a long period, internal lattice defects and dislocations develop; microscopic structures within the material to sustain internal stresses decrease; and the effective stress of the material increases. In this context, the increased effective stress of the test material in its aged state is the current stress of that material.
Moreover, whereas the conventional fatigue test method based on the relation between the coercive force and the effective tensile stress allows the maximum of the applied tensions to be only several to several tens times of the minimum, the test method according to the present invention determines fatigue of a test material based on the relation between the susceptibility coefficient c and the current stress "sgr", allowing the maximum stress "sgr" to be about 3000 times of the minimum, as seen from FIG. 10. This indicates that the method according to the present invention is more significantly sensitive to change in the tensile stress, which serves as a parameter for evaluating fatigue of a test material.
Incidentally, since it is known that there exists a simple relation between stress and the dislocation density of a material, it is possible roughly to estimate the dislocation density of the material from the tensile stress, to thereby nondestructively determine one of the factors representing fatigue of the material.
The method according to the present invention can be applied not only to ferromagnetic construction materials having a single crystal structure, but also to ferromagnetic construction materials having a polycrystalline structure, and to low-alloy steel. The present invention provides a highly sensitive, nondestructive test method for determining fatigue of ferromagnetic construction materials, which makes it readily possible to nondestructively determine the dislocation density of a ferromagnetic construction material and its distribution within the material even before cracks are generated in the material, and also to perform nondestructive measurement using only a small magnetic yoke and a small capacity power source.
In the nondestructive test method according to the present invention, the initial tensile stress "sgr"0 of the test material may be obtained from the following equation:
"sgr"0=F/Sxe2x80x83xe2x80x83(6)
where F represents a force applied to the test ferromagnetic construction material, and S the sectional area of the material normal to the direction of the force. In this instance, assuming that the external force and/or the internal force applied to the test material are known, the initial tensile stress "sgr"0 can be readily derived from the equation (6).
Alternatively, the initial tensile stress "sgr"0 of the test material may be obtained from the equations (1) and (2) in the same manner as is in the current tensile stress "sgr". In this instance, even when the external force and/or the internal force applied to the test material are unknown, the initial tensile stress "sgr"0 can be readily derived as is the case with the current tensile stress "sgr".
Still further, in the nondestructive test method according to the present invention, there may be used a U-shaped magnetic yoke for measuring the intensity H of a magnetic field applied to a test ferromagnetic construction material. It is then possible to perform a nondestructive measurement on the test material having a shape which does not readily permit a coil to be wound around it.